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Abstract
In this article, a compact racetrack double microring resonator (MRR) sensor based on Ge28Sb12Se60 (GeSbSe) is investigated. The sensor device consists of a racetrack microring, an embedded small microring, and a strip waveguide. Electron beam lithography (EBL) and dry etching are used to fabricate the device. The compact racetrack double MRR device are obtained with Q-factor equal to 7.17 × 104 and FSR of 24 nm by measuring the transmission spectrum. By measuring different concentrations of glucose solutions, a sensitivity of 297 nm/RIU by linear fitting and an intrinsic limit of detection (iLOD) of 7.40 × 10−5 are obtained. It paves the way for the application of chalcogenide glasses in the field of biosensing.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
Corrections
Zhiyong Li, Cheng Hou, Ye Luo, Wei Zhang, Lan Li, Peipeng Xu, and Tiefeng Xu, "Embedded racetrack microring resonator sensor based on GeSbSe glasses: erratum," Opt. Express 31, 33403-33404 (2023)https://opg.optica.org/oe/abstract.cfm?uri=oe-31-20-33403
1. Introduction
Chalcogenide glasses (ChGs) are amorphous materials formed by the covalent bonding of elements such as sulfur, selenium, and tellurium in Group VIA, with a certain amount of other metal or non-metal elements [1–3]. The structural characteristics of ChGs make them have many advantages, such as mid-infrared transmittance, low two-photon absorption, and high Kerr nonlinearity, and they can be deposited on any bottom without considering lattice mismatch [4–6]. Moreover, the preparation of chalcogenide device is completely compatible with the CMOS semiconductor micro-/nano-manufacturing process. Based on these outstanding merits, ChG devices are widely used in different fields, such as optical amplifiers [7], optical modulation [8], optical filters [9], optical sensors [10], and optical switches [11].
Optical sensors have many characteristics, such as fast reading, high stability, strong anti-electromagnetic interference ability, low cost, and compact structure [12,13]. They are highly valued in medical research [14], food safety [15], environmental monitoring [16], and other fields [17–19]. In recent years, versatile sensors based on microring resonator (MRR) [20], photonic crystal (PC) [21], Mach–Zehnder interferometer (MZI) [22], and subwavelength gratings (SWG) [23] have been proposed and investigated. MZI sensor mainly uses the optical path difference of asymmetric arms to realize sensing. Although high sensitivity can be achieved, the footprint of the MZI sensor is too large to facilitate integration. SWG and PC have high requirements and small error tolerance during manufacture because of their structural characteristics. The MRR sensor has excellent stability, with relatively simple manufacturing process, which provides a low limit of detection (LOD) and high sensitivity, due to its high quality factor (Q-factor) and small mode field volume.
In MRR sensor, a high Q-factor and large free spectral range (FSR) are desirable for better performance, but the high Q-factor and large FSR cannot be achieved simultaneously in a single MRR. A MRR with larger radius shows lower loss and higher Q-factor due to the small bending loss; however, the FSR is reduced [24]. Similarly, a decreased ring radius will increase FSR but degrade Q-factor for a higher bending loss. Therefore, researchers have put forward various schemes to ensure large FSR without sacrificing the Q-factor [25,26]. Regardless of the method used, the device should be compact and easy to fabricate, which is critical for a high-density integrated “lab-on-a-chip” system. Cascading dual rings are developed to increase Q-factor and FSR by Vernier effects; however, the manufacturing process is complex [27]. A dual ring using SOI has been reported to meet high Q-factor and wide FSR, which consists of a typical single ring and another small ring inside coupled with the outer ring. Because the virtual length and optical phase delay of the inner ring are improved by adding the outer ring [28], the performance can be further enhanced in the racetrack structure, due to the flexible control of the coupling length brought by the structure, and the enhancement in coupling efficiency [29,30].
In this study, a racetrack microring resonator based on Ge28Sb12Se60 (GeSbSe) film embedded MRR is proposed. By embedding a small microring into the racetrack microring and optimizing the structural parameters, electron beam lithography (EBL) and dry etching are used to fabricate the device, and a high Q-factor and large FSR are obtained by measuring the transmission spectrum, with a compact structure. Finally, as a proof of principle, we successfully exploit a refractive index (RI) sensing work for glucose detection with different glucose solution concentrations, and the sensitivity and LOD of the sensor are also evaluated.
2. Design and optimization of MRRs
2.1 Theoretical analysis
Figure 1(a) shows the schematic of the proposed MRR, which consists of a bus waveguide, a racetrackMRR, and an embedded ring resonator. For a MRR, the resonance condition is given by the following equation:
where λres is the resonance wavelength, neff is the effective RI, L is the length of the ring, and m is the resonance order (m = 1, 2, 3…). The transfer matrix method (TMM) is a common method to analyze the spectrum response of the MRR. The two red boxes in Fig. 1(c) represent the two coupling regions, in which we have
Fig. 1. Schematic configuration of the proposed MRR (a) 3D view; (b) cross-section view of the coupling region; (c) plane view
The transmission equation from one coupling region to another can be written as:
2.2 Parameter optimization
The MRR device is designed on a silicon substrate with a buried SiO2 layer. Figure 1(b) shows the cross-section view of the ChG layer. From previous simulation results [31,32], only the fundamental mode of TE/TM is effectively supported when the width of the waveguide is less than 750 nm, whereas the bending loss is negligible when the radius of the micro ring is greater than 5 µm. Therefore, the width of the waveguide is chosen as 600 nm, the radius of the outer ring (R1) is 20 µm, and the radius of the inner ring (R2) is chosen to be 5 µm.
The coupling length is an important factor in our design. Changing the coupling length can increase the coupling efficiency and also improve the process tolerance. The effect of changes in coupling length as a function of FSR is simulated, as shown in Fig. 2. From the figure, FSR increases as the coupling length increases, and it reaches a maximum value of 29 nm as the coupling length increases to 13 µm, after which it tends to remain constant.
Figure 3(a) compares the normalized transmission spectrum of the single and dual-MRR models obtained from 2.5D varFDTD solver (MODE Solutions) simulations. The green line represents the transmission spectrum of the embedded microring, and the red line is the transmission spectrum of a regular single ring with 20 µm radius, 200 nm gap, and 13 µm coupling length. The FSRs of a single and dual-MRR are calculated to be 5.9 and 29 nm, respectively, in the figure, which shows that the FSR of the dual-MRR is about five times as high as that of the single-MRR. Figure 3 (b) shows the electric field distribution of the proposed structure. As can be seen from the figure, the light is effectively coupled into the inner ring. In this case, the gap between the racetrack resonator and the bus waveguide (Gap1) is set as 200 nm, and the gap between the small ring and racetrack resonator (Gap2) is also set as 200 nm to attain a near-critical coupling operation. The parameters of the embedded MRR are as follows: R1 = 20 µm, R2 = 5 µm, Lc = 13 µm, Gap1 = 0.2 µm and Gap2 = 0.2 µm
Fig. 3. (a) Transmission spectrum of the designed device with R1 = 20 µm, R2 = 5 µm, Lc = 13 µm, Gap1 = 0.2 µm and Gap2 = 0.2 µm, and a single ring with the same parameter. (b) Electric field distribution of the proposed device.
3. MRR fabrication and characterization
3.1 Fabrication of MRR
Next, 300 nm-thick GeSbSe film is first deposited on 2 µm silicon dioxide, and the photoresist (arp6200) is spin-coated onto GeSbSe film by a spin coater. Then, the designed MRR structure is transferred onto the photoresist by EBL technology (Raith eLINE Plus), after which the device is baked on a heating plate before the inductively coupled plasma etching process (Oxford 100). The GeSbSe layer is primarily etched by the gas mixture of CHF3 and CF4, and then the sample is bombarded with the gas mixture of CHF3/O2/Ar to remove the residual photoresist. Finally, the device is placed in N-methyl-2-pyrrolidone solution and shaken for 10 min, before baking on a heating plate.
The SEM image of the fabricated ChG MRR is shown in Fig. 4(a). Figure 4(b) and Fig. 4(c) show the two coupling region of the racetrack ring and inner ring, respectively. Figure 4(d) shows the full-etched grating coupler for TE polarization with a period of 920 nm and a duty cycle of 0.788. The manufacturing error between the designed and fabricated structural parameters is about 40 nm. A smaller waveguide width will lead to a larger coupling gap, which will degenerate the coupling efficiency. Therefore, compensation is needed during the layout design before production to achieve the simulated structural parameters.
Fig. 4. (a) SEM image of the fabricated ChG MRR. (b and c) Two coupling regions of the racetrack ring and inner ring, respectively. (d) Full-etched grating coupler for TE polarization with a period of 920 nm and a duty cycle of 0.788.
3.2 Measurement and sensing experiments
The optical fibers are vertically coupled to the input and output of the device using grating couplers [33], which allows for easy coupling and high alignment tolerances [34]. The built-up test platform is used to examine device performance as shown in Fig. 5(a). A tunable laser emits a laser with a wavelength range of 1500 to 1600 nm with a tuning step of 0.005 nm, and a polarization controller is used to adjust the light to TE mode light. A single-mode fiber (SMF) is coupled into the MRR, and another SMF is connected to a power meter that displays the output transmission spectrum on a computer.
Fig. 5. (a) Schematic of the characterization setup. (b) Transmission spectrum of the ChG MRR. (c) Single resonance peak with a Lorentzian fit to Q-factor. Experimental transmission (black points) and Lorentzian fitting (red line) of transmission spectra.
Next, a RI sensing experiment of glucose solutions is carried out. Changes in the RI around the device result in a wavelength shift in the transmission spectrum. In our experiment, the concentration of glucose solution changes from 1% to 5% with a 1% step, and the device is placed on a thermoelectric cooler to maintain the temperature at 20 °C to prevent the influence of temperature fluctuations.
4. Results and discussions
4.1 Performances of ChG MRR
Figure 5(b) shows the output transmission spectrum of ChG MRR at wavelengths ranging from 1500 nm to 1600 nm. The FSR is measured to be about 24 nm, which is slightly smaller than the simulated one (29 nm), and the discrepancy may be related to the preparation process and dimensional errors of the device fabrication. Furthermore, the transmission spectrum is sensitive to the coupling factor, and the critical coupling is not satisfied when the coupling factor changes due to the coupling gap deviation caused by fabrication errors. The Q-factor can be obtained from Eq. (5):
where ΔλFWHM is the full-width-at-half-maximum of the resonant peak. Figure 5(c) shows the Lorentzian fitting result of one resonant peak near the wavelength of 1577 nm, in which ΔλFWHM is about 22 pm and the Q-factor is calculated to be proximally 7.17 × 104.4.2 Sensitivity and LOD of biosensing
Diabetes is one of the leading causes of death and disability in the world, which is a major cause of myocardial infarction, heart disease, coronary heart disease, and blindness. Therefore, highly sensitive and reliable glucose sensors become necessary. Based on this phenomenon, our proposed sensor is used for glucose sensing, in which the concentration varies from 1% to 5% in 1% steps. The corresponding RI can be found from the following equation [35]:
where n is the RI of the glucose solution and c is the concentration of the glucose solution. According Eq. (6), the RIs of 1% and 5% glucose are 1.331215 and 1.339275, respectively. Figure 6(a) shows the transmission spectrum shifting by measuring different concentrations of glucose solutions, which are dropped onto the surface of the device in sensing experiment.Fig. 6. (a) Measured transmission responses of MRR immersed in different concentrations of glucose solution. (b) Peak wavelength shifts as a function of RI change in glucose solution.
A red shift of the resonance peak with increasing solution concentration is observed, and the sensitivity of the microring sensor can be calculated from equation S = Δλres/Δnc, where Δλres is the resonance wavelength shift and Δnc is the RI variation in the cladding of the device. As shown in Fig. 6(b), the sensitivity is identified to be approximately 297 nm/RIU by linear fitting. LOD indicates the minimum RI change that the sensor can detect. The intrinsic LOD can be expressed as iLOD=λres/(Q·S), where λres is the resonance wavelength. From the experimental result, the corresponding iLOD is calculated to be 7.40 × 10−5 RIU.
Table 1 summarizes the traits of different structures of sensors on Si, Si3N4, and ChGs. A comparison of the proposed structures and MRR, photonic crystal, Bragg grating, and MZI is listed in the table. The sensitivities in Refs. [37] and [39] are slightly larger than that in the present study, but their Q-factors and iLOD are lower than those in the present study. In addition, both the sensitivity and iLOD in the present study are improved compared to those in our previous work with a single microring [32]. In short, our study shows advantages both in sensitivity and Q-factor and provides evidence for biomedical field application.
Table 1. Reported Q-factor and sensitivity based on different structures of sensor
5. Conclusion
In this article, we designed a compact RI sensor based on racetrack double MRR. By embedding a small MRR in a large racetrack MRR, high Q-factor and wide FSR can be obtained simultaneously. Furthermore, the racetrack MRR not only increases the coupling efficiency but also improves the process tolerance. Through the calculation of TMM, the transmission of the racetrack double MRR is analyzed, and the structure parameters are optimized in detail. Then, the sensor is fabricated using EBL and ICP etching techniques. The racetrack double MRR device are obtained with Q-factor equal to 7.17 × 104 and FSR of 24 nm. Finally, our proposed sensor is used as a biosensing application for glucose sensing, and a sensitivity of 297 nm/RIU and an iLOD of 7.40 × 10−5 are obtained with R1 = 20 µm. This study shows that ChGs have great potential in biosensing applications such as the detection of glucose concentrations in blood and urine.
Funding
Natural Science Foundation of Zhejiang Province (No. LD22F040002, No. LY23F050008); National Natural Science Foundation of China (No. 12104375, No. 62175202); the Natural Science Foundation of Ningbo (No. 202003N4007); the K. C. Wong Magna Fund in Ningbo University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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